Magnetic refrigeration technology at ambient temperature has been known for more than twenty years and the advantages it provides in terms of ecology and sustainable development are widely acknowledged. Its limits in terms of its useful calorific output and its efficiency are also well known. Consequently, all the research undertaken in this field tends to improve the performances of such a generator, by adjusting the various parameters, such as the magnetization power, the performances of the magnetocaloric element, the heat exchange surface between the heat transfer fluid and the magnetocaloric elements, the performances of the heat exchangers, etc.
The choice of the magnetocaloric materials determine and has a direct effect on the performances of a magnetocaloric heat generator. The magnetocaloric effect reaches its maximum in the neighbourhood of the Curie temperature of the magnetocaloric materials in their phase transition zone. Now, the materials have either a first-order phase transition and have a high magnetocaloric efficiency that is limited on a narrow temperature range around the Curie temperature, or they have a second-order phase transition with a lower efficiency, but on a wider temperature range. Now, a particular difficulty consists in producing a magnetocaloric generator that can operate with a high efficiency in a wire temperature range in order to adapt the temperature levels of the generator to the external temperatures.
Many magnetocaloric heat generators use the magnetocaloric effect of magnetocaloric materials by circulating a heat transfer fluid along or through the magnetocaloric materials, in two opposite directions, according to the magnetization and demagnetization cycles of the magnetocaloric materials. The used fluid is also intended for a thermal exchange of its calories and/or frigories with an external circuit. At the time of starting a heat generator using magnetocaloric material, the fluid circulation allows obtaining a temperature gradient between the opposite ends of the magnetocaloric material. Obtaining this temperature gradient depends on the initial temperature and on the flow rate of the heat transfer fluid, on the intensity of the magnetocaloric effect, on the Curie temperature and on the length of the magnetocaloric material. The closer the initial temperature and the Curie temperature of the magnetocaloric material, the faster a temperature gradient will be reached as from which the generator will be functional and able to produce or exchange thermal energy with an external circuit. But the initial temperature of the heat transfer fluid is not controlled and is equal to the temperature outside of the generator, and it thus can lie in a very wide range of temperature, for example between −20 and +60° C. In these conditions, the magnetocaloric materials must be chosen in function of their Curie temperatures and of the environment in which the generator will be integrated or will operate. To increase the efficiency, this requires, for a given application, to produce a generator for each environment type.
A solution to limit the number of specific generators to be provided consists in using magnetocaloric materials having a wide transition zone, namely materials with a second-order phase transition. But the magnetocaloric effect of these materials is low, and this limits the interest of this solution, since it does not allow obtaining an acceptable generator efficiency.
Another suggestion consists in integrating several magnetocaloric materials with first-order phase transition in the generator. However, this solution shows disadvantages linked with the small transition zone of these materials, since the magnetocaloric effect of some of these materials cannot take place if the temperature of the heat transfer fluid never reaches the transition zone of these materials. Furthermore, the time required to reach a temperature gradient between the hot and cold ends of the magnetocaloric element may be long because of the multiplicity of the materials used.